This invention relates to improvements in electric motors, and in particular to an improved method of testing the integrity of the motor and associated motor drive circuitry of an electrical power assisted steering system.
Electric motors are becoming increasingly common in a diverse range of applications. It is known, for example, to provide an electric power steering system of the kind comprising an input shaft, an output shaft, a torque sensor adapted to measure the torque in the input shaft, and an electric motor adapted to apply an assistance torque to the output shaft dependent upon the torque measured by the torque sensor.
A typical electric motor comprises a rotor, a stator and a number of phase windings on a yoke. Applying suitable voltages across each of the phase windings causes current to flow through the windings, generating a current flux vector in the air gap between the stator and the rotor. This current flux interacts with the magnetic field of the rotor to cause the rotor to rotate to a point of equilibrium in which the flux vector is aligned with the axis of the rotor magnetic field.
To cause the rotor to turn continuously, the current passed through the windings must be varied in a sequence. This causes the flux vector to rotate. This can be achieved by modulating the voltages across each winding under the control of a motor drive circuit.
According to a first aspect, the invention comprises a motor control strategy for use in combination with an electric motor said motor having a number of phase windings and said motor being adapted to produce an output torque in response to current in said windings comprising the steps of:
during normal operation generating a motor torque demand signal indicative of the output torque required from said motor, applying a first set of currents to said windings of said motor in response to said motor torque demand signal to produce a first output torque from said motor and during a test operation to enable diagnostics to be performed generating a motor current demand signal indicative of the total current required in said motor windings and adjusting said currents applied to one or more of said windings in response to both said torque demand signal and said motor current demand signal, whereby said adjusted currents produced in said windings are substantially equal to the total current demanded by said motor current demand signal regardless of the value of said motor torque demand signal and said motor produces a second output torque substantially equal to said first output torque produced during normal operation.
By total current we mean the algebraic vector sum of all the currents in the windings of the motor.
Thus, the method may comprise a test operation of controlling the total motor current to a predetermined required value without affecting the amount of torque produced by the motor. This allows a predetermined test current to be applied without altering the output torque characteristics of the motor, making the tests transparent to a user of the motor.
The motor may form a part of an electrical power assisted steering system of the kind set forth.
Preferably the test operation is performed on-line in real time, i.e. when the steering system is in use and the vehicle is running. It may be applied when the vehicle is moving or stationary. The total current in the motor may be held constant by the test operation, or may be varied.
The method may comprise the additional steps of using feedback control to control the currents in the motor windings.
Those skilled in the art to which the present invention relates will be familiar with the so-called xe2x80x9cvector controlxe2x80x9d or xe2x80x9cflux vector controlxe2x80x9d technique for electric motor control in which the manipulated variables are the direct (d-axis) and quadrature (q-axis) components of the motor current vector. Characteristic of the d-axis current is it direct alignment with the rotor magnetic field (which therefore generates zero motor torque). Characteristic of the q-axis current is its quadrature alignment with the rotor magnetic field which therefore generates maximum torque per ampere.
During normal operation the technique may comprise the steps of measuring the currents in one or more of the motor windings, measuring the motor rotor position, processing the measured currents in combination with the rotor position to produce a measured d-axis and a q-axis current component, processing the motor torque demand signal to produce a respective d-axis and q-axis current demand signal, converting the d-axis and q-axis current demand signals into a phase voltage demand signal for each winding; and applying a phase winding voltage across each winding in response to the respective voltage demand signal.
In an alternative technique, the motor current demand signal may be stored as a d-axis and q-axis component rather than a single motor current value, and so the step of processing the motor torque demand signal to produce the d-axis and q-axis values can be omitted as they are already in that form.
During the test operation, the method may further comprise the steps of adjusting the winding currents by generating an additional d-axis current demand signal. This is generated by processing the motor current demand signal in combination with the motor torque demand d-axis and q-axis current components. In this case, the q-axis and d-axis current demand signals used to generate the individual phase winding voltages will be equivalent to the normal q-axis current demand signal and the sum of the normal d-axis current demand value and the additional d-axis current component produced due to the test operation respectively.
The principle advantage of employing the non-torque producing component of motor current for diagnostic testing of the integrity of the motor and/or the motor drive circuitry is that the testing sequences can be designed in a non-intrusive fashion so that they do not affect or interrupt the normal running of the machine.
The non-intrusive nature of testing associated with the present invention may present the following advantages:
Diagnostic tests may be activated over a wider operating envelope, for example, during normal operation rather than just at power down, since changes in torque will not be apparent to the driver.
Diagnostic tests may be operated at substantially higher current levels where appropriate to improve the accuracy and/or reliability of the diagnostic tests than tests previously carried out using torque producing current.
The test routine may apply the adjusted current to the windings over a predetermined period of time t. It may further comprise the steps of performing one or more diagnostic test sub-routines during the test routine. The adjusted current may be kept constant throughout the predetermined period.
The test routine may be adapted for use in combination with a motor overload current detection means and may further comprise a motor overload current test sub-routine. The motor overload current test sub-routine may comprise the steps of generating a motor current demand signal which corresponds to a motor current in excess of a predetermined level. It is known to provide, as part of a motor drive circuit, a motor overload detection means which is adapted to produce an output indicative of an overload condition. For example, an error flag may be lowered (or raised) if the current in the motor exceeds the predetermined safe level. The test sub-routine may comprise the steps of testing the output of such an overload current detection means when the overload current is applied by the test routine.
The motor overload test routine may therefore comprise applying a simulated overload current to the motor by generating a suitable motor current demand value, and measuring the output of the motor overload current detection means during the simulated overload. If no overload signal is produced, i.e. the error flag is not lowered (or raised), the overload current test sub-routine may flag the overload detection circuit as inoperative.
The motor test routine may further comprise a sub-routine adapted to calculate the temperature of the motor.
It may comprise the steps of measuring the q-axis current and the q-axis voltage in the motor, measuring the d-axis current and the d-axis voltage in the motor, calculating the motor resistance from the algebraic vector sum of the voltage across the motor and the algebraic vector sum of the currents in the windings of the motor and estimating the motor temperature based upon the measured motor resistance. Performing these steps at elevated motor current and voltage levels ensures the accuracy and reliability of the calculated motor temperature and employing the present invention allows this to be done in a non-intrusive fashion.
The test routine may comprise a sub-routine adapted to test the contact resistance of a motor relay adapted to isolate the motor from a supply voltage.
The test routine may further comprise a sub-routine adapted to test the integrity of one or more motor drive stage filter capacitors. The filter test sub-routine may comprise the steps of generating a motor current demand corresponding to a predetermined current in the motor, isolating the one or more filter capacitors (of the drive circuit) from the supply voltage and measuring the rate of fall of voltage across the filter capacitors. Obviously, the filter capacitor should preferably be fully charged from the supply voltage before it is isolated and tested using the current in the motor as a load. The time taken for the voltage across the capacitor fall from a first voltage to a second voltage may be used as the basis of the test, with the time being compared to the time that would be expected for a correctly functioning filter. A smaller capacitance, perhaps due to partial breakdown of the dielectric, would loose its charge quicker than a healthy capacitor. Since motor current is kept constant during the test routine, this test can conceivably be performed at any time during operation of the motor.
In accordance with a second aspect, the invention provides an electrical power assisted steering system incorporating a motor and a motor drive circuit adapted to control the motor according to a control strategy of the first aspect of the invention.